1. Field of the Invention
The present invention relates to reading out capacities and in particular to reading out differential capacities.
2. Description of Prior Art
Today in many apparatuses differential capacities are used as sensitive elements. For example, differential capacities are used as electrical elements on various sensors. Such sensors comprise, for example, acceleration sensors, pressure sensors or path sensors. Today many circuit variations are known for reading out differential capacities, which, however, cannot be employed for specific problems or only with restrictions.
For example, in particular applications it is required to detect a relatively slow change in capacity with high resolution. At the same time, however, changes in capacity occurring for a short time must be recognized within a very short period of time during readout. Such requirements occur, for example, when a single sensor having an acceleration-dependent differential capacity is employed for traction control of a motor vehicle and is to serve as an airbag sensor at the same time. Such a sensor should be capable of recognizing a crash within a period of time that permits the release of an airbag before the passengers in the motor vehicle can be harmed on the one hand. At the same time, such a sensor should be capable of detecting the acceleration data that are significantly lower compared to the accelerations occurring in a crash with high accuracy to permit secure traction control.
Another example in which a sensor is to meet the requirements of both detecting slow changes in capacity with high accuracy and recognizing very fast changes includes the manipulation of heavy weights. Often, in particular applications such as demolition of houses, in which a crane controls the movement of an iron ball, a control of weights is carried out as a function of acceleration signals provided by a sensor fastened to the heavy weight. On the one hand, for such an application the very slow movement of the large mass should be detected by the sensor with high resolution in order to perform an exact maneuvering in a particular position. On the other hand, changes in acceleration by means of impacts which may occur when a large mass impinges on the wall of a house, for example, and which may be as strong as several 100.000 g should be recognized, the demands on such a sensor being significantly higher due to the high change in acceleration within a very short time interval than for the above-described case of a sensor intended to serve as traction control and simultaneously as an airbag sensor.
For the above-described requirements of a detection of a slow change in capacity of the differential capacity with a high resolution and the simultaneous recognition of very fast changes in capacity two or more sensors are known to be used, a first known sensor being used to perform the slow change in capacity with high accuracy and a second sensor being used to recognize the fast change in capacity. For reading out, a readout circuit which may be further adapted specifically to the requirements of the sensor is used for each sensor.
A known readout circuit is described in DE 19645028 A1, for example, in which a LC oscillator is used to detect a deflection of a middle plate of a differential capacitor. Using a switchover means, the partial capacities of the differential capacity are alternately switched as frequency-determining capacities into a LC oscillating circuit. The LC oscillating circuit comprises an oscillator transistor operated in a source circuit. The switchover means includes switching diodes, a frequency measurement being shifted by an appropriately selected time window in such a way that a frequency measurement is excluded during a transient state of the oscillator. Subsequently, the frequencies of the two circuits are determined and are provided to another interpretation for obtaining a deflection of the middle plate. The frequency measurement may be determined, for example, by measuring impulses falling into a predetermined time window, the values being averaged above a plurality of oscillation periods. However, the circuitry described above has the disadvantage that a continuous readout with high accuracy is not possible, because readout is not possible during a transient state of the oscillator. In addition to this, a secure readout of fast changes in capacity which occur for a short time during the transient states is thus not possible.
U.S. Pat. No. 4,860,232 discloses a circuit for measuring capacities, a reference capacity and a sensor capacity being connected to a voltage comparison means comparing an input voltage with a reference voltage. A feedback loop is used to generate an offset voltage which is proportional to the difference of the reference capacity and the sensor capacity. However, the use of the voltage comparison means has the disadvantage that complicated calibrations are necessary to compensate charge injections caused by a switchover.
U.S. Pat. No. 5,973,538 describes a sensor circuit in which a first arrangement of CMOS inverters connected in series, which is connected to a first capacitor, and a second arrangement of CMOS inverters connected in series, which is connected to a second capacitor, are used to generate output voltages indicating the capacity value of the first and the second capacitor.